Most tanks and vessels are manufactured in accordance with specific codes and standards, e.g., ASME Boilers & Pressure Vessel Code, DOT Code, AAR Code, and the like. To meet these standards, some vessels are manufactured by certain accepted methods. For thick-walled vessels, hollow cylindrical structures such as metallic tubes have been constricted at the ends to form vessels and tanks, such as high pressure tanks and fire extinguishers. One method of constricting the ends of hollow cylindrical structures to form high pressure tanks is by rotating the cylindrical structure, heating the end portions thereof and applying pressure on that heated end portions. For example, U.S. Pat. No. 2,699,596 (Aronson) discloses a process for making gas pressure cylinders by heating the side walls of a tube and spinning metal from the side walls into the bottom of the pressure vessel. Similarly, U.S. Pat. No. 2,408,596 (Bednar et al.) discloses a method for forming cylinder ends by torch-heating, rotating, and applying pressure to a cylindrical work piece. Pressure is applied by a tool moving in arcuate paths. U.S. Pat. No. 2,406,059 (Burch) discloses a process for spinning hollow articles suitable for closing the end of a tube. The end portion of the tube is heated by a heating means such as an oxy-acetylene flame. Pressure is applied by means of a flat-faced tool to the end portion of the tube to close it. Manfred Runge in "Spinning and Flow Forming," Verlag Moderne Industrie, 1994, discloses hot spinning to close thick-walled tubes for making high-pressure gas bottles. In such hot spinning on thick-walled tubes, induction coil is described as usable for preheating a tube. When spinning, gas burners are used for compensating for heat loss by the tube. Cold spinning using mandrels is also disclosed. Such a method can be used for making large thin-walled tank ends.
While spinning using mandrels can be employed to make thin-walled tank heads (or ends), such tank heads must be welded to each other or to a tube to result in a closed vessel since there is no good way of removing a mandrel from a closed vessel. Furthermore, in hot spinning a large, thin-walled structure, the relatively large surface area to volume ratio leads to rapid heat loss, thereby making it difficult to maintain temperature. Moreover, compressive stresses acting parallel to the surface of a thin-walled tube may bend, wrinkle, and collapse the tube because positive external pressure tends to buckle the surface. The resistance of the tube to buckling is proportional to a number ranging from the second to the third power of the tube thickness, depending on location along the tube and other factors. Thus, wrinkling and buckling is a severe problem in making thin-walled vessels. Techniques found to be useful for thick-walled vessels do not work on thin wall vessels. Forming such vessels by spinning without a mandrel is difficult.
Recently, U.S. Pat. No. 5,235,837 (Werner) discloses an apparatus for producing thin-walled cylindrical pressure vessels or tanks through metal spinning operations. The end caps of the vessels are formed from a hollow, thin wall cylindrical worktube. Forming rollers are moved along a plurality of arcuate stroking paths. The worktube is heated by heating torches. By controlled programming of the motion of the forming rollers, the forces applied to the worktube by the forming rollers, and the temperature of the tube, controlled distribution of the metal thickness in the knuckle zone can be accomplished. This method can provide greater thickness in the knuckle zone to strengthen it. As used herein, the term "knuckle zone" refers to the zone on the vessel at which the noncylindrical part is connected to the cylindrical part.
Unfortunately, flame heating can lead to oxidation and deterioration of the metallic tube. Methods have been devised to reduce the deterioration of steel in heat spinning processes. U.S. Pat. No. 3,594,894 (Mayer Jr.) discloses a method for forming a cartridge by heating a uniformly thick tubular material to a temperature slightly above the recrystallization temperature of the material and forming the material in dies heated to a temperature below the recrystallization temperature of the material. A heating means that may contain an inductive coil can be used to completely surround the ends of the tubular material and allow control of the tube temperature to a temperature slightly above its recrystallization temperature. A disk is used for sealing the end of the cartridge by welding. U.S. Pat. No. 3,964,412 (Kitsuda) discloses a shaping device in a circulation system for producing a high pressure gas container by successively drawing a workpiece at a series of workstations. The workpiece is mounted on a turn table and heated by a high frequency induction heater at a stop position after the first stop position or at any subsequent stop position where the workpiece can still be drawn.
Uniform heating, particularly of larger vessels, is difficult to achieve. Heating torches tend to concentrate the heat at the spots directly impinged by the flames. For heating larger tubes, many flame nozzles (or torches) will be needed. The iteration of these flame nozzles can lead to overheating and failure of adjacent nozzles. Further, open heating by flame nozzles is inefficient as a low percentage (5-10%) of the energy is transmitted to the workpiece while the rest is dissipated to the environment. If hotter but fewer flame nozzles are used, the hotter temperature will lead to accelerated deterioration of the metal. On the other hand, inductive heating has not been shown to be capable of effectively heating large metallic tubes for spinning, particularly those with large diameter to tube wall thickness ratios.